It has become common for users of host computers connected to the World Wide Web (the “web”) to employ web browsers and search engines to locate web pages having specific content of interest to users. A search engine, such as Microsoft's Live Search, indexes tens of billions of web pages maintained by computers all over the world. Users of the host computers compose queries, and the search engine identifies pages that match the queries, e.g., pages that include key words of the queries. These pages are known as a “result set.” In many cases, particularly when a query is short or not well defined, the result set can be quite large, for example, hundreds or thousands of pages or more. The pages in the result set may or may not satisfy the user's actual information needs. The vast majority of users is not interested in retrieving the entire huge set of resources, and rather is satisfied with a relatively limited number of authoritative results which are highly relevant to the topic of the query.
A number of search engines rely on many features in their ranking techniques. Sources of evidence can include textual similarity between query and documents or query and anchor texts of hyperlinks pointing to documents, the popularity of documents with users measured for instance via browser toolbars or by clicks on links in search result pages, and hyper-linkage between web pages, which is viewed as a form of peer endorsement among content providers. The effectiveness of the ranking technique can affect the relative quality or relevance of pages with respect to the query, and the probability of a page being viewed.